1. Field of the Invention
The present invention relates to a transparent coordinate input device that can be used over the screen of a display unit.
2. Description of the Prior Art
Various types of coordinate input devices are commercially available. They can be classified into pressure conductive-resistive type, static coupling type, and electromagnetic induction type devices in accordance with input position detection techniques.
A conventional static coupling type coordinate input device (e.g., Japanese Patent Laid-Open No. 48-69436) uses a resistor plate with a large number of x- and y-coordinate electrodes in a peripheral region thereof and an insulating plate formed on the resistor plate. An AC voltage is applied to the electrodes of the resistor plate along the x and y directions. When an operator touches any point of the insulating plate with an input pen, an AC voltage is induced by static coupling in the input pen and the x- and y-coordinates can be detected. The static coupling type coordinate input device requires a complicated detector since AC voltage is used. Furthermore, input detection errors tend to occur due to a body effect, thereby distorting the image on the display screen.
A conventional electromagnetic induction type coordinate input device uses a large number of x and y parallel conductor loops orthogonal to each other. When the operator touches any point with an electromagnetic input pen supplied with an AC current, a voltage is induced at the x and y conductor loops near the input point. A maximum value of the induction voltage is detected to determine the x- and y-coordinates of the input point. A conventional input device of this type also requires a complicated voltage detector. In addition, detection errors tend to occur due to holding the electromagnetic input pen at an improper angle, resulting in distortion on the display screen.
A conventional pressure conductive-resistive type coordinate input device (e.g., Japanese Patent Laid-Open No. 58-87676) uses an input structure obtained by sequentially stacking a square resistor film, a pressure conductive rubber sheet, and a metal conductive film. The square resistor film has a large number of x- and y-coordinate detection electrodes on the respective sides thereof. The pressure conductive rubber sheet is rendered conductive when pressure is applied. A constant current source is connected to the conductive layer and the electrodes. When the operator touches any point (i.e., an input point) of the conductive layer with an input pen, the portion of the pressure conductive rubber sheet corresponding to the input point is rendered conductive. At least one of the current components flowing to the detection electrodes of the opposing sides with respect to the input point is detected, so that the x- and y-coordinates of the input point can be detected. With a conventional coordinate input device of this type, the x- and y-coordinates of the input point can be easily detected as compared with the devices described above. However, the following drawbacks are also present.
An input graphic or character pattern is displayed on a display unit, such as a CRT, that is external to the coordinate display device. The operator must change the field of view from the input surface to the display screen to check whether or not the input pattern has been properly entered. Therefore, effective man-machine interface cannot be achieved, and input is cumbersome and time-consuming.
In a conventional pressure conductive-resistive type coordinate input device, a constant current from a single contact current source is shunted from the input point to a pair of opposing electrodes. One of the shunted current components is measured by a detection resistor and an operational amplifier. The current to be measured is supplied to a detection resistor having a resistance sufficiently smaller than that of the resistor film. A voltage drop across the ends of the detection resistor is amplified by the operational amplifier to measure current. In order to achieve an accurate measurement, the detection resistor cannot have a high resistance, and thus the voltage drop across the ends of the resistor is small. In this sense, the operational amplifier must have a large gain and is thus sensitive to external noise.
In a conventional coordinate input device of this type, coordinate detection precision is determined solely by the sheet resistance of the resistor film. When a uniform sheet resistance cannot be obtained, the input points are distorted on the display screen.